Material for the production of photographic prints conventionally comprises a sheet-form main substrate or support, photosensitive material on the support sheet and an overlying layer of protective material. The term "photosensitive material" as used herein includes those materials, which may be formed in a number of layers, which contribute to the formation of the visual image in the final print. The photosensitive material includes photosensitive emulsions, a number of which may be utilised in producing colour prints, and emulsions or other materials required for auxiliary functions in the forming of the image such as support for dyes, protective filters and so on.
The protective layer above-mentioned usually consists of hardened gelatin, possibly with the addition of other colloidal emulsion-formers or plastics, which are intended to improve the mechanical properties of the protective layer. Such protective layers occasionally also contain means for changing the texture and the reflecting power, e.g. matting agents.
Since such protective layers swell in water, they are sensitive to undesirable marks such as water stains, finger-marks, and so on.
The main substrate or support used in most cases for photographic printing materials is paper. Apart from the fact that it is cheap, paper has important advantages: it can readily be manufactured in different weights and with different textures. It can be coloured and have various coatings. However, paper also has disadvantages which have an adverse effect on its use as a support for photographic images.
With wet processes of the kind conventional in photography the paper loses its mechanical strength and dimensional stability for example. Because of its absorption it can absorb large quantities of chemical processing solutions so that there is a risk of chemicals being transferred from one bath to another during processing. Finally, paper prints usually have to be washed for a very long period to make them durable and as free from harmful chemicals as possible.
There are known steps adapted to obviate these disadvantages. For example, the paper used as an emulsion support for positive photographs can be replaced, for example, by an opaque non-absorbent plastics film, e.g. a white-pigmented triacetate or polyester film. Such materials are expensive and unsuitable for mass sales, however, in the thickness requires for photographic prints. Another method of producing non-absorbent image supports is to coat paper with a thin plastics film. Thus at the present time papers coated with polyethylene on both sides are being used in large quantities for the production of photographic prints. Support materials of this kind are relatively cheap, but they still have some disadvantages. For example, the edges of cut papers are of course unprotected. Chemicals can penetrate into the paper felt here and cause edge discoloration. In practice, the polyethylene surface is wetted unevenly; it is very difficult to produce uniform layers on such a surface without some cloudiness.
United Kingdom Patent Specification No. 355 303 (Saunders) issued in 1931 describes a process for the production of black and white positive photographs in which a black and white transparency is first produced in an emulsion carried on a conventional celluloid support and is cemented at the emulsion side to a reflecting support paper after processing and drying. This process avoids some of the above disadvantages of conventional paper prints; more particularly, the paper does not come into contact with chemical processing solutions and the outwardly facing celluloid film effectively protects the image from mechanical damage.
There is no appreciation in the Saunders specification that his process can have the advantage of avoiding contact between the paper and processing chemicals, nor has the Saunders process become established. One reason for this is the use of celluloid, which is conventional in the photographic industry, as a support for the photographic emulsions. Such supports are at least 60.mu. thick and relatively stiff. Because of their stiffness they are not easy to cement. Celluloid supports are also relatively expensive.
Another disadvantage of the process disclosed by the Saunders specification is that if laterally correct photographs are to be obtained exposure must either be through the celluloid support or laterally reversed directly on to the emulsion side. Exposure through the support results in lack of sharpness which increases with the support thickness. Laterally reversed exposures are unpopular in printing for various reasons. Cementing the image-bearing emsulsion layer to the paper support can result in a further loss of sharpness.
Printers have to keep in stock printing materials not only of different gradation but also of different surface types (textures) and thicknesses. This occupies a considerable amount of space and requires capital. The potential for easing this problem offered by the process disclosed in the Saunders specification has not been previously recognised.
One object of the present invention is to enable positive photographic prints to be produced without contacting the main paper support with processing chemicals while obviating the difficulties arising out of the process of Specification No. 355 303.